Heat exchange apparatus



April 29 1952 L. J. MARSHALL 2,594,471

HEAT EXCHANGE APPARATUS Filed oct. 11, 194e Patented Apr. 29, 1952 HEATEXCHANGE APPARATUS Leonard J. Marshall, Tenay, N. J., assigner toCombustion Engineering-Superheater, Inc., a

corporation of Delaware Application October 11, 1946, Serial No. 702,860

This invention relates to heat exchangers and particularly to thearrangements of economizers and air heaters for large steam generatingunits such as are used in the power stations of public utilities.

The usual arrangement of the large steam generating units comprises afurnace which is generally provided with steam generating tubes on itswalls for absorbing radiant heat from the burning fuel, followed, withrespect to the flow of the gases from the furnace, by convection heatabsorbing surfaces including steam generating boiler tubes placed eitherahead of and/or following a superheater in which the steam is heated, aneconomizer for heating the feed water to the generator, and an airheater for heating the air for combustion.

The air heater is generally selected of the largest practical size sinceit is a relatively inexpensive form of heat absorbing surface. Itprovides an air temperature leaving the heater which approaches theentering gas temperature and often the temperature difference betweenthe gas entering and the air leaving is less than 100 deg. F. As thisdifference becomes less, the heat absorption per unit area of heatingsurface diminishes and consequently the required amount of heatingsurface increases and approaches a practical limit. The temperature ofthe gas leaving the heater is generally determined by the desiredoverall eiiiciency of the steam generating unit. By selecting a maximumpractical sized air heater, the permissible entering gas temperature isestablished.

The economizer is selected to raise the temperature of the feed water tobelow that at which the water boils within the steam generator, which isthe saturation temperature of the steam. Customarily this water leavingtemperature is about 40 to 50 deg. F. below the saturated steamternperature at the normal operating loads of the steam generator. Inthis manner assurance is provided against generating steam in theeconomizer under the normal operating loads.

Having established the gas temperature entering the air heater andcalculating the heat absorbed by the water owing through the economizerwhen heated up to the desired temperature, the temperature of the gasentering the economizer is determined. From the calculations of the heatabsorbed from the burning fuel by the steam generating tubes in thefurnace and that absorbed from the gases leaving the furnace vby theboiler tubes ahead of the superheater and by the superheater tubes, thetemperature of 5 Claims. (Cl. 257-220) gases leaving the superheater isestablished. The difference in the temperatures between the gasesleaving the superheater and that of the gases entering the econornizeris a measure of the amount of heat to be absorbed from the gases byadditional steam generating boiler tubes.

'It is costly to provide a large number of such tubes since the numberand spacing of these tubes affects the diameter and thickness of thedrums `into which they connect and the drums constitute relatively largepercentages of the cost of high pressure steam generators. It istherefore desirable to reduce or even eliminate the steam generatingboiler tubes located between the superheater and the economizer in steamgenerating units, where such tubes are now required.

It is accordingly an object of this invention to provide improvedapparatus for reducing or even eliminating said steam; generating tubesVand therewith the, cost of the steam generating unit.

How the foregoing, together with other objects and advantages as mayhereinafter appear or are incident to my invention are realized, isillustrated in preferred form in the accompanying drawings wherein: I

Figure l is a vertical sectional view of a steam generating unitincluding heat recovery apparatus embodying the invention;

Figure 2 is a section taken on line 2-2 of Figure 1; and

Figure 3 is an end view of a damper control quadrant.

The embodiment of the steam generating unit illustrated includesthefurnace A in which fuel is burned and from which the products ofcombustion or gases pass successively through a superheater B, a boilerC, a first vair heater D, an economizer E and thence through a secondair heater F. The furnace A is of well known construction which may havesteam generating tubes lll and ll (here indicated schematically) mountedon the side and end walls respectively. Burners l2 may be locatedadjacent the corners or" the furnace A through which the fuel may beprojected into the furnace in such manner as to cause a turbulentvortical flame.

The boiler C comprises a bank of vertical tubes I3 (also shown singleline) which precede the superheater B with respect to the direction ofgas flow and a bank of tubes I4 which follow the superheater B. Bothbanks of tubes are connected to top and bottom drums I5 and l5.

Bafiles Il and I8 direct the gas now downwardlythrough superheater Bandl upwardly through bank M. A conduit I9 conducts the gases from theboiler C to the respective gas inlets and 2| of air heater D andeconomizer E. Conduit 22 receives the gases from the respective gasoutlets 23 and 24 of air heater D and economizer E and conveys them tothe gas inlet 25 of air heater F. Conduit 26 conveys the cooled gasesfrom the gas outlet of air heater F. Within conduit 22 may be provideddampers 21 and 28 which may control the eillux from the gas outlets 23and 24 of the respective air heater D and economizer E.

Air heaters D and F may comprise abundle of tubes 29 and 30 respectivelythroughA which the gases flow while the air passes over the outside ofsaid tubes. Air enters the inlet 3| ofair heater F through conduit 32.By means of partitions 33. 34 and 35 tube sheets 36 and 31 and conduits38, 39 and 40, the air is caused to flow in a sinuous path as shown bythe arrows, across the tubes of air heater F. A conduit 4l connects airoutlet 42 of air heater F with the air inlet 43 of air heater D. Bymeans of partition 44 and tube sheets 45 and 46 and conduit 41 the airis caused to flow through air heater D in a sinuous path as shown by thearrows. Conduit 48 is connected to the air outlet 49 of air heater D andconveys the heated air to the point of use.

Economizer E may comprise a multiplicity of.

tube coils 50 placed side by side to forma bundle of tubes across whichthe gases ow and through which feed water is conducted from` inletheader 5| thence through said coils through outle header 52.

In Figure 3 is shown a typical quadrant having lever 54 for adjustingdampers 21, the lever being connected to the shaft 55 of damper 21. Thedampers may be constructed to form a group of louvres extendingtransversely of the conduit 22, the louvres being interconnected by theusual connecting rods, not shown. Like damper 21, damper 28 isconstructed in similar manner and adjusted by a similar quadrant.

From the above it will be seen that the iiow of gases leaving the boileris divided-into two portions, one portion passing through the economizerE and the other portion passing through the air heater D. Thereafterboth portions join and pass through air heater F. It will be noted thatall of the air that passes through air heater F also passes through airheater D so that the ratio of air to gases is relatively increasedwithin air heater D. This apportionment of the air quantity will causethe metal or .tube temperatures of air heater D to be substantiallylower than they would be if all of .the-gases passed therethrough and Ihave found that forfv this reason I can deliver relatively hotter gasesto air heater D and yet maintain safe metal temperatures.

By delivering hotter gases to air heater D, which then absorbsrelatively more heattherefrom, it becomes possible to remove asubstantial part of and in certain circumstances all of heat absorbingtube surface I4 from the boiler C, which surface is now no longer neededto absorb said heat. The combined air heaters D and F will then absorb agreater amount of lheat than will a single air heater like Fif usedalone in the usual manner and yet the increase in surface of heaters Dland F over a single heater will not be proportional absorption. l

The economizer E, like air heater D,'receives but a portion of the gasesleavingthe boiler so to the increased f heat that the ratio of water togas is relatively high, and it therefore also may receive hotter gaseswithout danger of adding so much heat as to cause steaming. Because theeconomizer is now located in a hotter gas zone, its size will be smallerand its cost less. As in the case of air heater D, by delivering hottergases to the economizer E, it is now possible to remove a substantialpart of and in certain circumstances all of heat absorbing tube surfaceI4 from the boiler C. The lower drum. I6 will then reduce to the size ofa header, accommodating but a relatively few rows of boiler tubes.

Obviously the portions of the gases delivered to the air heater D andthe economizer E may be distributed to suit desired temperatures of airand water leaving these elements. Under certain conditions. economizer Emay be omitted, whereupon a portion of the gases may ow through by-pass56 around air heater D, or the economizer E may be replaced by otherheat exchanger.

In cases where low outlet gas temperatures from the air heater F can bejustified from the standpoint of increased over-all efficiency, it issometimes impossible to obtain suihcient air temperatures for milldrying of high moisture coals when using the single air heater. Thesuggested arrangement of two-stage air heater provides higher airtemperatures which can be then used for successful mill drying.

By-pass 56 controlled by damper 51, which may be adjusted like dampers21 and 28, may .be provided between the air heater D and economizer E.Such a by-pass may be advantageously used at low loads of the boilerwhen there is a possibility of the lower end of air heater F becomingtoo cold and thereby subject to corrosion due to condensation of themoisture in the gases. Damper 51 is then opened suiciently to permitsome of the hot gases to pass uncocled into air heater F and therebyraise the temperature of the entire heater and particularly the cold endthereof so as to prevent said condensation and incident corrosion.

The by-pass 5E and control damper 51, together with dampers 21 and 28may be used to advantage during the starting up of the steam generatingunit, by stopping all gas ow through the economizer E, so as to avoidsteaming therein and causing the gases to flow throughair heater D andby-pass 56. By increasing the normal flow of gas through the air heaterD at starting, relatively hotter air may be delivered from air heater Dto pulverizing mills for the purpose of drying the fuel.

While the preferred embodiment of my invention has been shown anddescribed, it will bc understood that minor changes in construction,combination and arrangement of parts may vbe made without departing fromthe spirit and scope of the invention as claimed.

What I claim is:

1. In heat exchange apparatus for transferring heat from hightemperature gases obtained from a given hot gas source; the combinationof a first and second air heater, each air heater having an air inletand an air outlet, a gas inlet and a gas outlet; a heat exchanger havinga gas inlet and a gas outlet; conduit means for delivering said hightemperature gas to both the gas inlet of said first air heater and thegas inlet of said heat exchanger; conduit means for delivering said hightemperature gas to the gas inlet of said second air heater therebyby-passing said rst air heater and said heat exchanger; conduit meansconnecting the gas outlet of both the rst air heater and the said heatexchanger to the gas inlet of the second air heater; conduit means fordelivering the air to be heated to the air inlet of the second airheater; conduit means connecting the air outlet of the second air heaterto the air inlet of the rst air heater; conduit means for removing theheated air from the rst air heater; and means for conducting a fluidthrough said heat exchanger for extracting heat from the portion of thesaid high temperature gases passing therethrough.

2. In apparatus for usefully reclaiming heat from the high temperaturegases obtained from a given hot gas source; the combination of aneconomizer for heating water; rst and second air heaters organized toaccomplish successive heating of air serially passed through bothheaters; means for flowing one portion of said high temperature gasesthrough said economizer whereby to reduce their temperature to anintermediate value Iby transfer of heat therefrom to said water; meansfor owing another `portion of said high temperature gases through saidfirst air heater whereby also to reduce their temperature to anintermediate value by transfer of heat therefrom to air coming into thatfirst heater from said second heater; means for flowing through saidsecond air heater said intermediatetemperature gases from the economizerplus said intermediate-temperature gases from the first air heaterwhereby to bring those combined oneportion and other portion combustiongases to a still lower temperature by transferring further heattherefrom to the unheated air entering that second heater; and means forforming a gas bypass around both the said economizer and the said firstair heater whereby to flow all the portions of said high temperaturegases over said second air heater at substantially the same temperatureat which said combustion gases would otherwise enter said economizer andsaid rst air heater.

3. In heat exchange apparatus for transferring heat from hightemperature gases obtained from a given hot gas source; the combinationof a rst and second air heater each air heater having an air inlet andan air outlet, a gas inlet and a gas outlet; a heat exchanger having agas inlet and a gas outlet; `conduit means for delivering said hightemperature gas to both the gas inlet of said first air heater and thegas inlet of said heat exchanger; conduit means for delivering saidvhigh temperature gas to the gas inlet of said second air heater therebyicy-passing said rst air heater and said heat exchanger; conduit meansconnecting the gas outlet of both the rst air heater and the heatexchanger to the gas inlet of the second air heater; conduit meansconnecting the air inlet of the second air heater to the air inlet ofthe first air heater; conduit means for conducting a ilow of air throughthe rst and second air heaters to be heated therein; and means forconducting a fluid through said heat exchanger for extracting heat fromthe portion of the gases passing therethrough.

4. In a heat exchange apparatus for transferring heat from hightemperature gases obtained from a given source; the combination of a rstair heater and a second air heater each heater having means forming agas space for flow of gas therethrough and means forming an air spacefor flow of air therethrough; a heat exchanger having means forming agas space for the flow of gas therethrough; means for conducting apredetermined portion of said high temperature gas through the gas spaceof said first air heater; means for conducting a predetermined portionof said high temperature gas through the gas space of said heatexchanger; means for conducting a predetermined portion of said hightemperature gas through the gas space of said second air heater therebyby-passing said first air heater and said heat exchanger; conduit meansconnecting the gas space of the rst air heater to the gas space of thesecond air heater, conduit means connecting the gas space of the heatexchanger to the gas space of the second air heater; means forconducting a flow of air through the air space of the rst and second airheaters to be heated therein; and means for conducting a uid throughsaid heat exchanger for extracting heat from the portion of said hightemperature gases passing therethrough.

5. In a heat exchange apparatus for transferring heat from hightemperature gases; the combination of a rst air heater and a second airheater each heater having a gas inlet and a gas outlet; a heat exchangerhaving a gas inlet and a gas outlet; means for conducting apredetermined portion of said high temperature gases to the gas inlet ofsaid first air heater; means for conducting a predetermined portion ofsaid high temperature gases to the gas inlet of said heat exchanger;means for conducting a predetermined portion of said high temperaturegases to the gas inlet of said second air heater thereby by-passing saidrst air heater and said heat exchanger; conduit means connecting the gasoutlet of the first airrheater to the gas inlet of the second airheater; conduit means connecting the gas outlet of the heat exchanger tothe gas inlet of the second air heater; conduit means for conducting aow of air through the rst and secondair heaters to be heated therein;and means for conducting a 'uid through said heat exchanger forextracting heat from the portion of the gases passing therethrough.

LEONARD J. MARSHALL.

REFERENCES CITED The following references are of record in the iile ofthis patent:

UNITED STATES PATENTS Number Name Date 2,075,044 Lucke Mar. 30, 19372,217,512 Donley Oct. 8, 1940 2,386,188 Artsay Oct. 9, 1945 2,392,325Kuhner Jan. 8, 1946 2,418,815 Baver Apr. 15, 1947 FOREIGN PATENTS NumberCountry Date 429.797 Great Britain June 6. 1935

